The present invention relates to a heavy duty pneumatic tire, more particularly to a bead structure capable of preventing breakage of carcass cords occurring in the vicinity of the axially inner end of the bead core.
In general, heavy duty pneumatic tires for trucks, buses and the like are provided with a carcass reinforced by steel cords extending between bead portions and turned up around a bead core in each of the bead portions from the axially inside to the axially outside of the tire. The heavy duty pneumatic tires have a high probability of being used under extremely severe service conditions with heavy loads and very high tire inflation pressure. Therefore, there is a possibility that the steel carcass cords are broken in the vicinity of the bead core.
Conventionally, in order to prevent the carcass cords from being broken by direct contacts with the steel wire of the bead core, the bead core is wrapped with a high modulus rubber layer for example as disclosed in U.S. Pat. No. 7,604,032 and Japanese patent application publication No. 2007-230400A.
In recent years, on the other hand, in view of environment improvements, tire manufactures make great efforts to reduce tire weight even in the heavy duty pneumatic tires for trucks, buses and the like.
In order to reduce the tire weight, for example, the number of carcass ply is reduced to one (single ply), and the volume of rubber such as bead apex, reinforcing layer, filler etc. used in the bead portions is decreased as disclosed in U.S. Pat. No. 7,604,032.
As a result, the motion of the carcass ply during running is increased in the case of such a heavy duty pneumatic tire (hereinafter “heavy duty pneumatic tire with slim beads”). Therefore, it becomes difficult for the conventional high modulus rubber layer to fully prevent the carcass cords from contacting with the steel wire of the bead core, and thus it is difficult to provide bead durability.